# Plan: Revision-Write Path (`AddRevisionValuesBegin/Value/End`)

Status: **NOT STARTED.** Sub-plan extracted from `speculative-items-sweep.md`
item D2 because the work is too large for a one-push sweep.

## Context

The Historian's "revision write" path is the documented mechanism for
editing historized data after the fact (replaces the inferred
`ModifyData` / `DeleteData` use cases that don't exist as WCF ops). Native
managed surface (per Phase 1 findings of the write-commands plan):

| Public method | Token | Purpose |
|---|---|---|
| `ArchestrA.HistorianAccess.AddRevisionValuesBegin` | `0x06006175` | Open a revision-edit transaction |
| `ArchestrA.HistorianAccess.AddRevisionValue` | `0x06006176` | Append a value to the open transaction |
| `ArchestrA.HistorianAccess.AddRevisionValuesEnd` | `0x06006177` | Commit the transaction |
| `ArchestrA.HistorianAccess.AddRevisionValues` | `0x0600617F` | Single-shot variant |
| `ArchestrA.HistorianAccess.AddVersionedStreamedValue` | `0x0600616F` | Push one versioned value (related path) |

WCF surface is unknown — likely a new op group on `IHistoryServiceContract2`
or `IRetrievalServiceContract4` or a new contract.

## Goal

Public SDK API:

```csharp
public Task<HistorianRevisionTransaction> BeginRevisionAsync(string tag, CancellationToken ct);
// On the returned transaction:
public Task AddRevisionValueAsync(HistorianSampleEdit sample, CancellationToken ct);
public Task<bool> CommitAsync(CancellationToken ct);
// IDisposable / IAsyncDisposable for cancellation rollback if such a thing exists
```

Or a single batch convenience:

```csharp
public Task<bool> WriteRevisionsAsync(string tag, IReadOnlyList<HistorianSampleEdit> samples, CancellationToken ct);
```

The choice depends on the wire shape — if Begin/Value/End requires the
caller to maintain a server handle between calls, the disposable
transaction is necessary; if it's stateless, the batch convenience is fine.

## Workstreams

### A. Static analysis (1-2 hours)

- Inspect IL for the four managed public methods to identify the
  underlying `CHistoryConnectionWCF.*` calls and their server-side WCF
  contract methods.
- Add the contract methods to `Wcf/Contracts/IHistoryServiceContract2.cs`
  (or a new contract if appropriate) with `[OperationContract(Name = "...")]`
  + `[MessageParameter]` attributes once names are known.

### B. Native harness extension (2-3 hours)

- Add `--scenario revision-write` to the harness.
- Refer to existing `--scenario write` plumbing for the AddTag wrapper
  pattern.
- Sequence:
  1. Open connection (probably write-enabled mode `0x401`)
  2. AddTag for sandbox tag (re-uses existing harness flow)
  3. AddStreamedValue for the initial sample (currently blocked
     architecturally per Phase 2 findings — but may not be required if
     the revision path operates directly on the historian engine state)
  4. AddRevisionValuesBegin / AddRevisionValue × N / AddRevisionValuesEnd
  5. Read back via existing read path; verify the samples reflect the
     edits

### C. Wire capture (1 hour)

- Same `instrument-wcf-writemessage` + `instrument-wcf-readmessage`
  IL-rewrite tooling already used for EnsT2 / DelT.
- Capture both Begin/Value/End and the single-shot AddRevisionValues
  variant for byte-level diff.

### D. Decode + managed serializer (4-6 hours)

- Walk the captured InBuff bytes against the native serializer IL.
- The Begin payload likely seeds a server-side transaction handle that
  Value calls reference. Look for an `out`-returned handle in the Begin
  response.
- Value payload structure is likely similar to `AddS2`'s pBuf
  (uint16 version + uint32 sampleCount + N × {tagId, FILETIME, quality,
  typed value bytes}) but may include a per-sample revision/version field.

### E. Public API + tests (4-6 hours)

- New types: `HistorianSampleEdit` (sample + reason/version metadata),
  `HistorianRevisionTransaction` (disposable handle).
- Public methods on `HistorianClient` per the Goal section.
- Unit tests: golden-byte fixtures for Begin/Value/End/Commit payloads.
- Live integration tests: write a known sample, edit it via the
  revision path, read back and assert the new value appears.

## Risks

- **Server-cache prerequisite.** If the historian's revision path
  also requires the tag to be "live in the runtime cache" (the same
  blocker that killed `AddS2`), the entire path may be unimplementable
  for the same architectural reason.
- **State across calls.** Begin/Value/End may store transaction state
  on the server keyed by the WCF session GUID. WCF's session model
  needs to be configured to keep the same channel alive across all
  three calls — which is a different lifecycle from the existing
  one-call-per-channel pattern in the SDK orchestrators.
- **Concurrent edits.** Server may reject concurrent revision
  transactions on the same tag — needs probing.
- **Time bounds.** Revision likely respects the same `RealTimeWindow`
  / `FutureTimeThreshold` system parameters as `AddS2`. Out-of-window
  edits silently drop or error — needs probing.

## Success Criteria

- Public `BeginRevisionAsync` (or batch variant) live-verified against
  a sandbox tag created by `EnsureTagAsync`.
- Round-trip test: write initial value → revise it → read back → verify
  the revised value persists in `History` extension table via SQL.
- Golden-byte fixtures for Begin / Value / End / Commit captured against
  the sandbox tag.
- Decision documented for whether the `AddRevisionValues` single-shot
  variant is exposed in addition to the Begin/Value/End sequence.

## Dependencies

- Existing analog write surface (`EnsureTagAsync`) — done.
- `AddS2` is **not** a prerequisite; the revision path may be an
  independent code path that bypasses the runtime-cache gate. If it
  doesn't, this plan is blocked the same way `AddS2` is.

## Out of scope

- Editing event tags. Events come from AVEVA AnE; the SDK only reads
  them.
- Bulk schema changes. Forbidden over the wire per the Historian's
  architecture.

## Trigger to start

A customer-driven request, or a real need to expose historical data
correction in the SDK's API. Without one, this remains the most
substantive remaining write-path workstream but isn't worth the 1-2
days of focused work speculatively.